Add DefaultEngine constant and replace semantic-default uses of CopilotEngine (#24554)

* Initial plan

* Define DefaultEngine constant and update usages from CopilotEngine where used as default

Agent-Logs-Url: https://github.com/github/gh-aw/sessions/847c0feb-cf66-4748-81c1-dd045e07daec

Co-authored-by: dsyme <7204669+dsyme@users.noreply.github.com>

* Update pkg/workflow/agentic_engine.go

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update pkg/cli/add_command.go

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: dsyme <7204669+dsyme@users.noreply.github.com>
Co-authored-by: Don Syme <dsyme@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 3 people

pkg/cli/add_command.go

@@ -395,9 +395,9 @@ func addWorkflowWithTracking(resolved *ResolvedWorkflow, tracker *FileTracker, o
 
 	// Handle engine override - add/update the engine field in frontmatter before source so
 	// the engine declaration appears above the source field in the final file.
-	// Copilot is the default engine, so we skip adding it to avoid unnecessary noise and
-	// prevent conflicts during later workflow updates.
-	if opts.EngineOverride != "" && opts.EngineOverride != string(constants.CopilotEngine) {
+	// The default engine is omitted to avoid unnecessary noise and prevent conflicts during
+	// later workflow updates.
+	if opts.EngineOverride != "" && opts.EngineOverride != string(constants.DefaultEngine) {
 		updatedContent, err := addEngineToWorkflow(content, opts.EngineOverride)
 		if err != nil {
 			if opts.Verbose {

pkg/cli/add_interactive_engine.go

@@ -23,7 +23,7 @@ func (c *AddInteractiveConfig) selectAIEngineAndKey() error {
 
 	// Determine default engine based on existing secrets, workflow preference, then environment
 	// Priority order: flag override > existing secrets > workflow frontmatter > environment > default
-	defaultEngine := string(constants.CopilotEngine)
+	defaultEngine := string(constants.DefaultEngine)
 	workflowSpecifiedEngine := ""
 
 	// Check if workflow specifies a preferred engine in frontmatter
@@ -52,12 +52,12 @@ func (c *AddInteractiveConfig) selectAIEngineAndKey() error {
 		}
 
 		// Priority 2: If no existing secret found, use workflow frontmatter preference
-		if defaultEngine == string(constants.CopilotEngine) && workflowSpecifiedEngine != "" {
+		if defaultEngine == string(constants.DefaultEngine) && workflowSpecifiedEngine != "" {
 			defaultEngine = workflowSpecifiedEngine
 		}
 
 		// Priority 3: Check environment variables if no existing secret or workflow preference found
-		if defaultEngine == string(constants.CopilotEngine) && workflowSpecifiedEngine == "" {
+		if defaultEngine == string(constants.DefaultEngine) && workflowSpecifiedEngine == "" {
 			for _, opt := range constants.EngineOptions {
 				envVar := opt.SecretName
 				if opt.EnvVarName != "" {

pkg/constants/constants_test.go

@@ -107,6 +107,9 @@ func TestAgenticEngines(t *testing.T) {
 	if string(GeminiEngine) != "gemini" {
 		t.Errorf("GeminiEngine constant = %q, want %q", GeminiEngine, "gemini")
 	}
+	if DefaultEngine != CopilotEngine {
+		t.Errorf("DefaultEngine = %q, want CopilotEngine (%q)", DefaultEngine, CopilotEngine)
+	}
 }
 
 func TestDefaultGitHubTools(t *testing.T) {

pkg/constants/engine_constants.go

@@ -20,6 +20,10 @@ const (
 	CodexEngine EngineName = "codex"
 	// GeminiEngine is the Google Gemini engine identifier
 	GeminiEngine EngineName = "gemini"
+
+	// DefaultEngine is the default agentic engine used when no engine is explicitly specified.
+	// Currently defaults to CopilotEngine.
+	DefaultEngine EngineName = CopilotEngine
 )
 
 // AgenticEngines lists all supported agentic engine names.

pkg/workflow/agentic_engine.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 	"sync"
 
+	"github.com/github/gh-aw/pkg/constants"
 	"github.com/github/gh-aw/pkg/logger"
 )
 
@@ -455,9 +456,9 @@ func (r *EngineRegistry) IsValidEngine(id string) bool {
 	return exists
 }
 
-// GetDefaultEngine returns the default engine (Copilot)
+// GetDefaultEngine returns the default engine configured by constants.DefaultEngine
 func (r *EngineRegistry) GetDefaultEngine() CodingAgentEngine {
-	return r.engines["copilot"]
+	return r.engines[string(constants.DefaultEngine)]
 }
 
 // GetEngineByPrefix returns an engine that matches the given prefix